Abstract:

Threaded connection between a male part (2) and a female part (1),
particularly between a screw and a dental implant, both parts (1, 2)
fitted with a thread profile with spires, whereby the thread step (4b) of
the thread profile of the male part (2) is different to the thread step
(4a) of the thread profile of the female part (1), obtaining a controlled
gap (3) between both parts. According to the invention, one of the two
parts (1, 2) has a variable thread step. Therefore, the threaded
connection between both parts (1, 2) presents a gap (3) that varies in a
non-linear manner. By appropriately adjusting the specific design of the
thread steps, it is possible to control the stress distribution
throughout the threaded connection in accordance with the interests of
the application.

Claims:

1. Threaded connection between a female part (1) and a male part (2),
whereby both parts (1, 2) comprise a thread profile made up of a series
of spires, where the thread profile of at least one of the two parts (1,
2) has a variable thread step (4a, 4b) so that a growing gap (3) is
created between both thread profiles, characterized in that:the
difference between the thread step of the parts (1, 2) is non-constant in
accordance with the spire number, in such a way that the gap (3) varies
in a non-linear manner.

2. Threaded connection, according to claim 1, characterized in that the
difference between the thread step of parts (1, 2) follows a (n-1)-grade
polynomic function, so that the gap (3) varies according to a n-grade
polynomic function.

3. Threaded connection, according to claim 2, characterized in that the
difference between the thread step of parts (1, 2) follows a linear
function, in such a way that the gap (3) varies parabolically (grade 2).

4. Threaded connection, according to claim 2, characterized in that the
difference between the thread step of parts (1, 2) follows a parabolic
function (grade 2), in such a way that the gap (3) varies elliptically
(grade 3).

5. Threaded connection, according to claim 1, characterized in that the
male part (2) has a constant thread step (4b) and the female part (1) has
a variable thread step (4a).

6. Threaded connection, according to claim 1, which is characterized by
the fact that the male part (2) has a variable thread step (4b) and the
female part (1) has a constant thread step (4a).

7. A method comprising connecting, by means of a threaded connection
according to claim 1, a dental implant and a connection screw for
connecting another piece to the dental implant, whereby the dental
implant is the female part (1) and the screw is the male part (2).

Description:

FIELD OF THE INVENTION

[0001]The invention relates to a threaded connection between a threaded
male part and a threaded female part, applicable to all types of threaded
connections, such as, for example, threaded connections between a dental
implant and screw, the latter being used to retain an artificial tooth,
etc.

PRIOR ART

[0002]Conventional threaded connections between a male part (with a thread
profile made up of a series of spires) and a female part (also with a
thread profile made up of a series of spires, and in which the thread
profile of the male part fits perfectly) have been designed so as not to
present gaps, or in other words, so that the thread profiles of both
parts fit each other perfectly. After analyzing these connections, the
conclusion has been reached that the stress distribution on the spires
during the useful life of the threaded connection makes some spires
withstand greater tension than others. More specifically, the spires that
withstand greater stress are the first spires, or in other words, the
spires closest to the head of the screw, or what is the same, furthest
away from the tip of the screw.

[0003]Some patents such as U.S. Pat. No. 3,664,540 and U.S. Pat. No.
2,870,668 are known, which describe threaded connections in which there
is a gap between the thread profile of the external part (male) and the
internal part (female) and the thread profile of the female part. The gap
grows linearly in order to homogeneously distribute stress, or in other
words, so that all of the spires withstand approximately the same
tension.

[0004]However, this linear-growing gap does not allow controlling the
stress distribution throughout the threaded connection, which would be
interesting for certain applications in which threaded connections with
different stress distributions would be desirable. In these certain
applications, neither a traditional threaded connection (in which the
first spires are under the most stress) nor a threaded connection as in
U.S. Pat. No. 3,664,540 or U.S. Pat. No. 2,870,668 (in which all spires
are under the same stress) might be the most appropriate. Instead, in
these certain applications it will be interesting that the stress peak
occurs in any of the spires of the threaded connection, even in the final
spires. For example, in threaded connections used in aeronautics, the
final spires must be the most burdened in order to ensure that the
majority of the screw remains (and therefore continues working) in the
event of breakage of the screw, ensuring that the threaded connection is
maintained. Alternatively, it may also be interesting to design the
threaded connection so that faults occur in the head of the screw and
hence facilitate the extraction of the screw in the event that it breaks.

[0005]The invention aims to offer a design for a threaded connection which
is appropriate for applications in which it is not necessary for the
stress distribution to be homogeneous nor do the first spires need to be
under the most stress.

BRIEF DESCRIPTION OF THE INVENTION

[0006]The object of the invention is a threaded connection of a male part
and a female part, both with a thread profile made up of spires, whereby
the thread of the thread profile of the male part is different to the
thread of the thread profile of the female part, there being a controlled
gap between both parts. According to the invention, one of the two parts
(male or female) presents a variable thread step. Therefore, the threaded
connection between both parts presents a gap that varies in a non-linear
manner (unlike the gaps in U.S. Pat. No. 3,664,540 and U.S. Pat. No.
2,870,668, in which the thread step of both parts is slightly different
but always constant, creating a gap that varies linearly). By
appropriately adjusting the specific design of the thread steps (and
therefore the type of non-linear gap), it is possible to control the
stress distribution throughout the threaded connection in accordance with
the interests of each specific application. Therefore, the invention
allows configuring or choosing the area of the threaded connection that
is going to be under more stress, and therefore allows controlling in
which area of the connection breakage will occur.

[0007]The variable thread step may either be in the male part or in the
female part. It must be taken into account that if it is applied in the
male part, it is generally an increasing thread step whilst if it is
applied to the female part, it is generally a decreasing thread step. The
other part will have a thread profile with a constant thread step.

[0008]It should be born in mind that the terms "increasing" and
"decreasing" are used observing the threaded connection from the head
towards the tip of the male part (the male part shall generally be a
screw or similar).

[0009]Preferably, the gap with non-linear variation presents a
substantially n-grade parabolic, elliptic or polynomic variation. The
choice of one type of gap or another depends on the application in which
the invention is going to be used.

BRIEF DESCRIPTION OF THE FIGURES

[0010]The details of the invention can be appreciated in the accompanying
figures, which do not intend to limit the scope of the invention:

[0017]FIG. 7 shows a graph of the distribution of forces throughout the
spires in a conventional threaded connection without a gap, in a
conventional threaded connection with a linearly-increasing gap, and in
the threaded connection according to the invention shown in FIG. 2.

[0018]FIG. 8 shows a table detailing gap values of a conventional
connection and of several embodiments of the invention.

[0019]FIG. 9 shows the graphic representation of the gaps in the previous
figure.

DETAILED DESCRIPTION OF THE INVENTION

[0020]FIG. 1 shows a conventional threaded connection between a female
part (1) and a male part (2), in which there are no gaps between both
parts (1, 2).

[0021]FIG. 2 shows an embodiment of the threaded connection according to
the invention, between a female part (1) and a male part (2), in which
gaps (3) between both parts (1, 2) are formed as a result of the fact
that the thread step (4) of the female part (1) and the thread step (4)
of the male part (2) are different. According to the invention, the
thread step (4) of one of the two parts (1, 2) increases from one spire
to another in an inconstant manner. Therefore, the width of the gaps (3)
increases in a non-linear way. In embodiment shown in the figure, it is
the female part (1) that presents the increasing thread step (4) whilst
the male part (2) presents the constant thread step (4). `Thread step
(4)` is understood to be the distance between one spire and the following
spire in the thread profile.

[0022]FIG. 3 shows an enlarged view of various consecutive spires of the
threaded connection between the female part (1) and the male part (2).
The thread step (4b) of the male part (2) is fixed, whilst the thread
step (4a, 4a', 4a'') of the female part (1) increases in successive
spires. Preferably, thread steps (4a, 4a', 4a'', 4b) are such that the
width of the gaps (3) presents a substantially parabolic, elliptic or
n-grade polynomic variation.

[0023]For the gap (3) to grow in a substantially parabolic manner, the
thread step (4b) of the male part (2) is constant whilst the thread step
(4a, 4a', 4a'') of the female part (1) follows a linear function with
respect to the spire number; i.e., the difference between a thread step
and the previous thread step is a constant value. In other words, the
graphic representation of the thread step of the female part (1) in
accordance with the spire number is a straight line. For example, if the
thread step (4b) of the male part (2) is a constant value `p`, the thread
steps (4a, 4a', 4a'') of the female part (1) are p+k, p+2k and p+3k
respectively, where k is constant. Therefore, in this case the width of
the gaps (3, 3', 3'') is k, 3k and 6k respectively. The complete gap
width series, for a high number of spires, would be k, 3k, 6k, 10k, 15k,
21k, 28k . . . , or in other words, a width that grows in a substantially
parabolic manner in accordance with the spire number, as seen in FIG. 9.

[0024]For the gap (3) to grow in a substantially elliptic manner, the
thread step (4b) of the male part (2) is constant whilst the thread step
(4a, 4a', 4a'') of the female part (1) follows a parabolic function with
respect to the spire number. For example, the thread step of the female
part (1) grows in such a way that the second thread is a constant value
longer than the first; the third is twice as long as the second and so
on. More specifically, if the thread step (4b) of the male part (2) is a
constant value `p`, the thread steps (4a, 4a', 4a'') of the female part
(1) may be p+k, p+3k and p+6k respectively. Therefore, the width of the
gaps (3, 3', 3'') obtained is k, 4k and 10k respectively. The complete
gap width series, for a high number of spires, would be k, 4k, 10k, 20k,
35k, 56k, 84k . . . , or in other words, a width that grows in a
substantially elliptic manner in accordance with the spire number, as
seen in FIG. 9.

[0025]For the gap (3) to grow in a substantially n-grade polynomic manner,
the thread step (4) of the male part (2) is constant whilst the thread
step (4a, 4a', 4a'') of the female part (1) follows a (n-1)-grade
polynomic function with respect to the spire number. For example, for a
gap (3, 3', 3'') whose width grows according to a polynomial with a grade
of 5, the thread step (4a, 4a', 4a'') of the female part (1) must grow
according to a polynomic function with a grade of 4. Looking at specific
values as an example, if the thread step (4b) of the male part (2) is a
constant value `p`, the thread steps (4a, 4a', 4a'') of the female part
(1) may be p+k, p+5k and p+15k respectively. Therefore, the width of the
gaps (3, 3', 3'') obtained is k, 6k and 21k respectively. The complete
gap width series, for a high number of spires, would be k, 6k, 21k, 56k,
126k, 252k, 462k . . . , or in other words, a width that grows in a
substantially 5-grade polynomic manner in accordance with the spire
number, as seen in FIG. 9.

[0026]Logically, the cases of parabolic growth and elliptic growth are
specific cases of the n-grade polynomial growth, whereby n=2 and n=3
respectively.

[0027]FIG. 4 shows the stress distribution throughout the conventional
threaded connection of FIG. 1, indicating a scale of the stress values
(MPa) different grey levels. As can be seen in the figure, the high
stress levels (shaded almost in white) are concentrated in the first
spires (left hand side of the

[0028]Figure) of the male part (screw). Then, following a relatively
abrupt transition (shown in different shades of grey) there are a large
number of loosely tensed spires (in dark grey and black) located in the
tip of the male part. The maximum stress value withstood by the male part
is approximately 752 MPa.

[0029]FIG. 5 shows the stress distribution throughout the threaded
connection of FIG. 2. In this case, the invention leads to several
interesting effects. Firstly, all spires are uniformly stressed, which is
appreciated because the transition between levels of grey is smoother
than in FIG. 4. Secondly, the maximum value or stress peak applied to the
male part is reduced to a value of around 568 MPa. In other words,
compared with the threaded connection in FIG. 4, the maximum stress value
applied to the male part (bottom part in this and other figures) is
reduced by approximately 20%.

[0030]FIG. 6 shows a graph that represents the maximum stress values
withstood by the threaded connection (in N/mm2) with respect to the
`k`-parameter (in mm) described above, by means of which the growth of
the thread step is parameterized. Two curves are displayed: a continuous
curve related to a threaded connection with a parabolically-increasing
gap according to the invention, and a dashed curve related to a threaded
connection with a linearly-increasing gap in accordance with U.S. Pat.
No. 3,664,540 or U.S. Pat. No. 2,870,668. Both connections aim to select
the optimum `k` value, which is that in which the maximum stress
withstood by the threaded connection is minimum. It is also observed that
the threaded connection with a parabolically-increasing gap generally
presents lower maximum stress values than a threaded connection with a
linearly-increasing gap, this difference being greater for larger `k`
values. Therefore, using a threaded connection according to the invention
with an appropriate `k` value, it is possible to obtain a significant
reduction in the maximum stress compared with the connections in U.S.
Pat. No. 3,664,540 and U.S. Pat. No. 2,870,668.

[0031]FIG. 7 shows a graph of the distribution of contact forces (in N)
withstood by each spire throughout the spires that make up the threaded
connection (indicated by the spire number). Three curves are displayed: a
continuous curve related to a conventional threaded connection without
gaps; a dashed curve related to a conventional threaded connection with
linearly-increasing gaps in accordance with U.S. Pat. No. 3,664,540 or
U.S. Pat. No. 2,870,668; a dash-dot curve related to a threaded
connection according to the invention with parabolically-increasing gaps.
As shown in the figure, in the conventional threaded connection without a
gap, the first spire absorbs an extremely high force (around 275 N), and
the force rapidly decreases in the following spires. In the threaded
connection with a linearly-increasing gap, the first spires receive a
lower burden, and the maximum force is applied on the ninth and tenth
spires, with a maximum value of around 175 N. However, in the threaded
connection according to the invention, all spires are loaded in a uniform
manner, which is beneficial because it reduces the risk of any of the
spires breaking, which usually happens when some spires carry a much
greater burden than others. Additionally, the maximum force value
withstood is far lower than in the two conventional threaded connections;
in fact, it is around 80 N, which corresponds to a reduction of 70% and
55% respectively compared with the two conventional threaded connections.

[0032]FIG. 8 shows a table that represents the increasing gaps obtained
according to several polynomic growth series: grade 1 (linear growth,
known in prior art), 2 (parabolic growth), 3 (elliptic growth), 4, 5 and
6. FIG. 9 shows the graphic representation of these gaps with respect to
the spire number. This graphic representation helps observe the
parabolic, elliptic, etc. growth of the gaps according to the invention.

[0033]The threaded connection of the present invention can be manufactured
using any conventional applicable procedure such as mechanization,
injection, lamination, etc., only taking into account that it must be
possible to create a non-constant thread (which is considered perfectly
achievable with any of these methods). The manufacturing procedure does
not require special tools or a longer production time than the
manufacturing procedure of a conventional threaded connection.

[0034]On another note, it must be pointed out that there are not any
optimum thread step increase parameters. Optimum thread step parameters
will depend on the thread dimensions, the thread material, the load to
which the threaded connection is subjected, etc. Depending on the needs
of each application, different optimum parameters will be calculated.

[0035]In a particularly beneficial way, the threaded connection according
to the invention is applied to the connection between a dental implant
and a connection screw of the dental implant, whereby the dental implant
is the female part (1) and the screw is the male part (2). The part that
is going to be connected to the dental implant shall normally be a pillar
post, but it may also be any other prosthetic component, such as a
cicatrisation pillar, an UCLA pillar, etc. In any case, the external
connection according to the invention provides a more robust mechanical
set (implant, screw and prosthetic component) compared to mechanical sets
provided in conventional threaded connections. Therefore, the robustness
of the set is increased without having to use different materials to
those normally used and without having to increase the manufacturing cost
of the different elements comprised in the mechanical set.